Dispersed optical heterodyne detected birefringence and dichroism of transparent liquids

The frequency-dispersed birefringence and dichroism of an electronically nonresonant liquid excited and probed by ultrafast pulses in an optically heterodyned detected configuration is reported. The nominally putative dichroic response of a transparent sample is shown to result from pi out-of-phase contributions of Stokes and anti-Stokes third-order polarization components on, respectively, the red and blue sides of the probe pulse spectrum and are derived from CSRS and CARS type resonances. The strong corresponding nondispersed birefringent response, in contrast, results from the in-phase combination of these Stokes and anti-Stokes polarization components. Thus, by observing the dispersed probe pulse to the red or blue of the central carrier frequency, the various density matrix components, or pathways in time evolution history, contributing to either a dichroic or birefringent measurement may be more selectively viewed. The contribution of a particular nuclear response is enhanced when the observed frequency within the probe pulse spectrum is tuned to be either one quantum to the red or blue of the carrier frequency. This frequency-filtering technique can be used to enhance weak features in the total response, allow the determination of isotropic and anisotropic contributions to a nuclear response, and help probe the homogeneous and inhomogeneous character of the low-frequency Raman active density of states commonly observed in these two-pulse responses of nonresonant liquids. These effects are illustrated by the OHD birefringence and dichroism of CHCl3.